1. Field of the Invention
The present invention relates to a zoom lens, which is suitable for use in a broadcasting television (TV) camera, a video camera for image pickup, and a digital still camera or the like, and to an image pickup apparatus including the zoom lens.
2. Description of the Related Art
Conventionally, there has been known, as a zoom lens for broadcasting television (TV) camera, a zoom lens in which a front lens group including a magnification-varying lens unit (magnification-varying portion) and a rear lens group including a relay lens unit (relay portion) are arranged. The front lens group is located on an object side of an aperture stop and the rear lens group is located on an image side thereof. The front lens group includes, in order from the object side to the image side, a first lens unit having a positive refractive power and including a focusing lens unit, a second lens unit having a negative refractive power for magnification-varying, and a third lens unit having a negative refractive power for correcting an image plane variation due to magnification-varying.
The rear lens group includes a fourth lens unit (relay portion) having a positive refractive power for imaging. In such a four-unit zoom lens including the four lens units in total, the relay portion includes two lens units separated from each other by a long air interval. There has been known a four-unit zoom lens in which, for example, a focal length converter (extender) for increasing a magnification (focal length) of the entire system may be inserted between the two lens units (Japanese Patent Application Laid-Open No. 2009-122418, U.S. Pat. No. 6,545,818).
In Japanese Patent Application Laid-Open No. 2009-122418, a ⅓-inch image pickup element is used. In U.S. Pat. No. 6,545,818, a ⅔-inch image pickup element is used. Japanese Patent Application Laid-Open No. 2009-122418 and U.S. Pat. No. 6,545,818 disclose a zoom lens for TV broadcasting in which the extender is removably insertable between, of the lens units included in the relay portion, lens units having a maximum air interval to shift a focal length range of the entire system.
In the zoom lens for a television camera, a color separation prism is disposed on the image side to obtain image data of R, G and B each. In a television camera main body side, the image data of R, G and B are synthesized into a single image to obtain a color image.
The zoom lens for a television camera has a long back focus to dispose the color separation prism on the image side. In order to suppress occurrence of white-shading (phenomenon that even when white balance is achieved in center of screen, upper end and lower end of screen are colored with green or magenta), an exit pupil is set as far away as possible from the image plane.
In many cases, the white-shading occurs because an incident angle of a ray entering a dichroic film of the color separation prism changes depending on a position on a screen. When the white-shading occurs, the quality of the color image significantly deteriorates over the entire screen.
In recent years, a small-size image pickup element has been used for the broadcasting TV camera because of the demand for reducing the size of the entire apparatus.
In the four-unit zoom lens described above, if the size of the screen is fitted to the small-size image pickup element, the power of the relay portion increases, and hence it is difficult to maintain the long back focus. Further, the position of the exit pupil which is a conjugate image of the aperture stop becomes closer to an image pickup element position (image plane).
As a result, it is difficult to dispose the color separation prism on the image side and the white-shading is more likely to occur. In recent years, as the zoom lens for a television camera, there has been strongly desired a zoom lens in which even when the small-size image pickup element is used, the back focus is sufficiently long and the exit pupil is set away from the image plane.